1. Field of the Invention
This invention relates to a diversity system in a portable radio for maintaining the high quality of received signals. A radio telephone apparatus or the like is a typical example of a portable radio of this type.
2. Description of the Prior Art
Many prior art portable radios have been provided with extendable monopole antennas. Monopoles, dipoles and other electric-field-type antennas are likely to be adversely affected by the human body; and if they are used in proximity to the human body the antenna's gain is lowered. Consequently, prior art portable radios of this type have the drawback that, when a radio is worn or placed in a pocket during use, the gain of the monopole antenna, even if extended, is lowered, thereby degrading reception.
Some prior art portable radios have been provided instead with magnetic-field-type antennas such as loop antennas whose gain is improved by the influence of a human body, eventually avoiding the above stated drawback. These radios include pagers, portable radio telephones and the like. This type of portable radio has the drawback that when it is placed on a table or otherwise not worn by the user, the gain of the antenna will be reduced, thereby degrading reception.
So-called mobile communications involve a complex propagation having multiple propagation paths generated by reflection of radio waves from buildings and other obstacles between a transmitter and a receiver by scattering and refraction from inhomogeneities in the atmosphere. A standing wave is likely to be produced at the point of reception. Movement over even a slight distance causes a variation of field strength ("fading") which can be approximated by a Rayleigh distribution. This particular fading is generally called Rayleigh fading and is characterized by an occasional great decrease in field strength. Diversity is frequently used to achieve good communication in the presence of Rayleigh fading. In space diversity, for instance, two or more identical types of antennas spaced more than one quarter wavelength from each other receive signals which are then combined or selected to diminish the effects of the fading phenomenon. The space diversity system, of course, has been difficult to apply to portable radios because of the requirement for distance between antennas.